Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The conventional starter/controller, referred to hereinafter as simply a starter or a controller, uses solid state switches for controlling application of AC line voltage to the motor. The switches may be thyristors such as silicon controlled rectifiers (SCRs) or triacs.
One known motor controller in the form of an elevator starter provides motor protection in the form of an overload and current imbalance fault. The current imbalance fault is logged as a single phase fault. Experience has forced the ratio of lowest current to the highest current to be reduced to 30% for a fault to occur. One of the conditions that forces the ratio to the low level is voltage imbalances on systems where one or two of the incoming phases are heavily loaded. In these circumstances the power company attempts to maintain the loaded phases at the nominal value. This can cause excessive voltages in the unloaded phases. If the motor is running, then these excessive voltages will cause high currents in the lightly loaded phase or phases. Under conditions where the load on the motor is light, causing the motor currents to be less than the overload settings, then the imbalance is completely ignored by the starter. Under conditions where the voltage is high enough on one or two of the incoming phases, then the currents can be high enough to exceed the overload setting and cause damage to the motor. Depending on the magnitude of the voltage, the iron associated with that motor winding can saturate which leads to very high currents. Allowing the starter to run under these conditions can cause excessive heating in the windings and reduce the lifetime of the motor. However, shutting the motor controller down under these conditions can cause the elevator to stop and result in service calls.
Other motor controllers monitor line voltage and limit the voltage on all three phases during high voltage conditions. On a system that works off all three voltages under situations where one voltage is extremely high the firing angle to the SCRs for all three phases is reduced. While the voltage to the motor would be decreased on the phase with the excessive voltage, the voltage would also be reduced on the other phase or phases. This may result in unbalanced currents and could cause the motor to go into a stall condition.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.